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Women With Dense Breasts More Likely to Develop Cancer in Opposite Breast

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Dense breasts have less fatty tissue and more non-fatty tissue compared to breasts that aren't dense. Dense breasts have more gland tissue that makes and drains milk and supportive tissue (also called stroma) that surrounds the gland. Breast density can be inherited, so if your mother has dense breasts, it's likely you will, too.

Research has shown that dense breasts:

  • can be twice as likely to develop cancer as nondense breasts
  • can make it harder for mammograms to detect breast cancer; breast cancers (which look white like breast gland tissue) are easier to see on a mammogram when they're surrounded by fatty tissue (which looks dark)

One way to measure breast density is the thickness of tissue on a mammogram.

The Breast Imaging Reporting and Database Systems, or BI-RADS, which reports the findings of mammograms, also includes an assessment of breast density. BI-RADS classifies breast density into four groups:

  • Mostly fatty: The breasts are made up of mostly fat and contain little fibrous and glandular tissue. This means the mammogram would likely show anything that was abnormal.
  • Scattered density: The breasts have quite a bit of fat, but there are a few areas of fibrous and glandular tissue.
  • Consistent density: The breasts have many areas of fibrous and glandular tissue that are evenly distributed through the breasts. This can make it hard to see small masses in the breast.
  • Extremely dense: The breasts have a lot of fibrous and glandular tissue. This may make it hard to see a cancer on a mammogram because the cancer can blend in with the normal tissue.

Still, no one method of measuring breast density has been agreed upon by doctors. Breast density is not based on how your breasts feel during your self-exam or your doctor's physical exam.

A study suggests that women who’ve been diagnosed with breast cancer and have dense breasts are nearly twice as likely to develop cancer in the opposite breast.

The research was published online on Jan. 30, 2017 by the journal Cancer. Read the abstract of “Mammographic breast density is associated with the development of contralateral breast cancer.”

About 43% of women ages 40 to 74 years old in the United States are considered to have dense breasts.

When a woman who’s been diagnosed with breast cancer develops breast cancer in the other breast, doctors call it "contralateral breast cancer."

"We know there are a number of well-established influences for developing both primary and secondary breast cancers, such as BRCA mutations, family history, and the tumor's estrogen receptor status," said Isabelle Bedrosian, M.D., associate professor of breast surgical oncology at MD Anderson who was one of the study’s authors. "We also know density is a risk factor for the development of primary breast cancer. However, no one has closely looked at it as a risk factor for developing contralateral disease."

To do the study, the researchers looked at the records of 229 women who were diagnosed with stage I to stage III breast cancer who were then diagnosed with cancer in the opposite breast more than 6 months after the initial diagnosis. The researchers then compared these women, called the "cases," to 451 women who were diagnosed with stage I to stage III breast cancer but did not develop cancer in the opposite breast. The women without cancer in the opposite breast were called the "controls." All the women were treated at the University of Texas MD Anderson Cancer Center between 1997 and 2012.

The researchers matched the case women with the control women on a number of factors, including:

  • age
  • year of diagnosis
  • hormone receptor status of the initial cancer

They then compared the breast density of the matched cases and controls. The women’s breast density was determined at the time of the first diagnosis.

Among women who were diagnosed with breast cancer in the opposite breast:

  • 39.3% had non-dense breasts
  • 60.7% had dense breasts

Among women who didn’t develop breast cancer in the opposite breast:

  • 48.3% had non-dense breasts
  • 51.7% had dense breasts

After the researchers adjusted the data to take into account other known breast cancer risk factors that the women had, they found that women with dense breasts had nearly twice the risk of developing contralateral breast cancer than women with non-dense breasts.

This difference was statistically significant, which means that it was likely due to the difference in breast density and not just because of chance.

"Our findings have valuable implications for both newly diagnosed patients with dense breasts and for breast cancer survivors as we manage their long-term risk of a secondary diagnosis," said Carlos Barcenas, M.D., assistant professor of breast medical oncology and the study’s corresponding author. "Our future goal is to develop a risk model incorporating breast density to best assess a breast cancer survivor's risk of developing contralateral breast cancer."

If you’ve been diagnosed with breast cancer in one breast and you know you have dense breasts, it makes sense to talk to your doctor about this study. Ask your doctor about all your options for reducing your risk of contralateral breast cancer. Together you can decide on a treatment plan that makes the most sense for you.

It also makes sense to talk to your doctor about developing a specialized surveillance program that meets your individual needs and gives you peace of mind.

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